Devices for the testing of electrical circuit boards are known for example from U.S. Pat. No. 3,564,408 and U.S. Pat. No. 4,417,204 respectively. These devices have a contact board on which test probes are arranged in a basic grid. The test probes are connected to a test circuit over long cables. The circuit boards to be tested are placed on the test board, and an adapter may be fitted between the circuit board and the test board so that an electrical contact is made between each test point of the circuit board to be tested and a test probe.
From this type of tester, modular testers have been developed, such as those described in patents DE 32 40 916 C2 and DE 33 40 180 C1. This type of tester has a motherboard on which rest vertically arranged modules, each comprising a part of the electronic test circuit and having vertically aligned test probes at their upper ends. In a tester, several such modules are mounted alongside one another, with the array of test probes forming a contact array replacing the contact board. So that the modules hold together well, a perforated board may be slipped over the test probes, with each test probe passing through a hole in the perforated board, thereby being fixed in position.
This modular structure of the contact array has proven to be very successful in practice. A major advantage of this modular structure is that the contact pressure applied in the testing of a circuit board is transmitted via the modules to the motherboard.
Known from utility-model patent DE 88 06 064 U1 is a further tester in which the contact array is modular in form. These modules are strip-shaped with each having, for example, four rows of square pads or contact faces. Here it is disclosed that the pads are arranged in a grid with a grid dimension of, for example, 0.5-2 millimeters (mm). These modules with such a dense arrangement of pads have not proved successful in practice since, on the one hand, owing to the high number of contact points the can only be read by very large and therefore expensive evaluation electronics, and on the other hand the embodiment described in DE 88 06 064 with a vertically placed circuit board, on the end face of which the contact points are formed directly, gives considerable problems in series production.
EP 0 875 767 A2 discloses a tester which is provided with evaluation electronics with a number of test connections. This tester has a basic grid formed by a multilayer circuit board with contact points arranged in a grid on its upper side. Several of these contact points are electrically connected to one another by means of scan channels running in the circuit board of the basic grid. On the basic grid rests an adapter and/or a translator on which a circuit board to be tested may be placed. The adapter and/or translator makes an electrical contact between circuit board test points on the circuit board and the contact points of the basic grid.
EP 1 322 967 B1 discloses another module for a tester for the testing of circuit boards. This has a strip-shaped section with contact points, which forms part of a basic grid of the tester. Arranged below the strip-shaped section is an upright board on which are mounted some of the evaluation electronics for the evaluation of test signals. The contact points on the strip-shaped section are arranged in a grid with a grid spacing of no more than 2 mm, and at least two contact points of a module are electrically connected to one another in such a way that the electrically connected contact points are in contact with a single input of an electronics unit.
Described in EP 1 083 434 A2 and U.S. Pat. No. 6,445,173 B1 respectively is another device for the testing of circuit boards, in which the basic grid is formed by circuit boards standing at right-angles to the plane of the basic grid. These circuit boards, which are described as basic grid circuit boards have, on their narrow-side faces lying in the plane of the basic grid, contact points which represent the contact elements of the basic grid. These contact points are connected to conductor paths which extend over one side face of the basic grid circuit boards. These basic grid circuit boards are in the form of multilayer circuit boards in which conductor paths running at right-angles to one another are arranged at different levels to form a matrix. At the crossovers of these conductor paths running at right-angles to one another they may be connected electrically by means of a via hole. Using these via holes and the matrix, selected contact points of the basic grid may be connected electrically with one another, so that the contact points connected to one another require only a single connection to evaluation electronics. Since the conductor paths of this matrix are distributed over a fairly large area of the basic grid circuit boards, they are much more easily provided than the scan channels known from EP 0 875 767 A2 referred to above.
Formerly the maximum density of the contact points of a basic grid of a commercially-available tester was 62 contact points per square centimeter, corresponding to a square grid with an edge length of 1.27 mm.
It is true that there are also testers on the market with a higher contact point density (double or four times). To date, however, these have been technically very complex and therefore expensive, and consequently have been made only in very low volumes.
For testers with basic grids, the parallel testers, the density of basic grid contact points should be further increased. This makes the production of such basic grids difficult, in particular if certain contact points of the basic grid are to be electrically interconnected. In the case of the basic grid circuit boards known from EP 1 083 434 A2, the production of such closely adjacent contact points on the narrow-side faces of the basic grid circuit boards is difficult. In the case of the modules with basic grids described above, with contact points which are electrically interconnected, it is difficult to implement the electrical connection of selected contact points if a further increase in contact point density is desired.
If for example it is desired that the density of contact points be doubled (64 contact points per square centimeter), this is not possible using the known methods. Therefore, to form the basic grid according to EP 0 875767 A2, it would be necessary to provide a basic grid circuit board with 48 layers. It is not possible to make such a circuit board reliably today using current production methods. It is virtually impossible for such a circuit board to have all its connections made correctly. A defective basic grid, however, is not acceptable in a tester.
A further problem which arises when the density of contact points in a basic grid is increased is the increasing mechanical pressure load on the basic grid. The pressure is proportional to the number of contact points. If the contact points are doubled, there is a corresponding increase in pressure.